Carbon nanotubes are hexagonal networks of carbon atoms forming seamless tubes with each end capped with half of a fullerene molecule. They were first reported in 1991 by Sumio Iijima who produced multi-layer concentric tubes or multi-walled carbon nanotubes by evaporating carbon in an arc discharge. They reported carbon nanotubes having up to seven walls. In 1993, Iijima's group and an IBM team headed by Donald Bethune independently discovered that a single-wall nanotube could be made by vaporizing carbon together with a transition metal such as iron or cobalt in an arc generator (see Iijima et al. Nature 363:603 (1993); Bethune et al., Nature 363: 605 (1993) and U.S. Pat. No. 5,424,054). The original syntheses produced low yields of non-uniform nanotubes mixed with large amounts of soot and metal particles.
Presently, there are three main approaches for the synthesis of single- and multi-walled carbon nanotubes. These include the electric arc discharge of graphite rod (Journet et al. Nature 388: 756 (1997)), the laser ablation of carbon (Thess et al. Science 273: 483 (1996)), and the chemical vapor deposition of hydrocarbons (Ivanov et al. Chem. Phys. Lett 223: 329 (1994); Li et al. Science 274: 1701 (1996)). Multi-walled carbon nanotubes can be produced on a commercial scale by catalytic hydrocarbon cracking while single-walled carbon nanotubes are still produced on a gram scale by laser techniques.
Generally, single-walled carbon nanotubes are preferred over multi-walled carbon nanotubes because they have fewer defects and are therefore stronger and more conductive than multi-walled carbon nanotubes of similar diameter. Defects are less likely to occur in single-walled carbon nanotubes because multi-walled carbon nanotubes can survive occasional defects by forming bridges between unsaturated carbon valances, while single-walled carbon nanotubes have no neighboring walls to compensate for defects. Defect-free single-walled nanotubes are expected to have remarkable mechanical, electronic and magnetic properties that could be tunable by varying the diameter, number of concentric shells, and chirality of the tube.
Single-walled carbon nanotubes have been produced by simultaneously evaporating carbon and a small percentage of Group VIII transition metal from the anode of the arc discharge apparatus (Saito et al. Chem. Phys. Lett. 236: 419 (1995)). Further, the use of mixtures of transition metals has been shown to increase the yield of single-walled carbon nanotubes in the arc discharge apparatus. However, the yield of nanotubes is still low, the nanotubes can exhibit significant variations in structure and size between individual tubes in the mixture, and the nanotubes can be difficult to separate from the other reaction products. In a typical arc discharge process, a carbon anode loaded with catalyst material (typically a combination of metals such as nickel/cobalt, nickel/cobalt/iron, or nickel and transition element such as yttrium) is consumed in arc plasma. The catalyst and the carbon are vaporized and the single-walled carbon nanotubes are grown by the condensation of carbon onto the condensed liquid catalyst. Sulfur compounds such as iron sulfide, sulfur or hydrogen sulfides are typically used as catalyst promoter to maximize the yield of the product.
A typical laser ablation process for producing single-walled carbon nanotubes is disclosed by Andreas Thess et al. (1996). Metal catalyst particle such as nickel-cobalt alloy is mixed with graphite powder at a predetermined percentage, and the mixture is pressed to obtain a pellet. A laser beam is radiated to the pellet. The laser beam evaporates the carbon and the nickel-cobalt alloy, and the carbon vapor is condensed in the presence of the metal catalyst. Single-wall carbon nanotubes that do not have constant diameters are found in the condensation. However, the addition of a second laser to their process which give a pulse 50 nanoseconds after the pulse of the first laser favored the 10, 10 configuration (a chain of 10 hexagons around the circumference of the nanotube). The product consisted of fibers approximately 10 to 20 nm in diameter and many micrometers long comprising randomly oriented single-wall nanotubes, each nanotube having a diameter of about 1.38 nm.
Unlike the laser and arc techniques, carbon vapor deposition over transition metal catalysts tends to create multi-walled carbon nanotubes as the main product instead of single-walled carbon nanotubes. However, there has been some success in producing predominantly single-walled carbon nanotubes from the catalytic hydrocarbon cracking process. Dai et al. (Chem. Phys. Lett 260: 471 (1996)) demonstrate web-like single-walled carbon nanotubes resulting from disproportionation of carbon monoxide (CO) with a molybdenum (Mo) catalyst supported on alumina heated to 1200° C. The diameter of the single-walled carbon nanotubes generally varies from 1 nm to 5 nm and could be controlled by the Mo particle size. The electron microscope images of the product shows the Mo metal attached to nanotubes at their tips. Rope-like bundles of single-walled carbon nanotubes have been generated from the thermal cracking of benzene with iron catalyst and sulfur additive at temperatures between 1100-1200° C. The synthesized single-walled carbon nanotubes are roughly aligned in bundles and woven together similarly to those obtained from laser vaporization or electric arc method. The use of metal catalysts comprising iron and at least one element chosen from Group V (V, Nb and Ta), VI (Cr, Mo and W), VII (Mn, Tc and Re) or the lanthamides has also been proposed (U.S. Pat. No. 5,707,916).
The presently available methods of synthesizing carbon nanotubes produce bulk amounts of carbon nanotubes that are generally tangled and kinked. Further, the nanotubes can have molecular level structural defects that can adversely impact their properties. Thus, the existing methods cannot fabricate a carbon nanotube at a pre-selected location. For example, one potential application of carbon nanotubes is as interconnect wiring within a circuit. U.S. Pat. No. 6,574,130 discloses a hybrid memory cell in which each cell has a nanotube ribbon crossbar junction. The nanotube ribbon is formed separately from a matted or tangled nanotube, and then placed at the desired location on the memory cell. This process could be simplified if methods for efficiently producing individual carbon nanotubes at discrete locations were available.
A method for manufacturing carbon nanotubes as elements of microelectromechanical manufacturing systems (MEMS) devices is disclosed in U.S. Pat. No. 6,146,227. A nanosize hole or nanoscale catalyst retaining structure is fabricated in the layer on the MEMS substrate into which a nanotube growth catalyst is deposited. A nanotube is then grown within the nanosize hole. The method thus controls the location and size of the nanotubes by placing a nanoscale catalyst retaining structure at a specific location on the MEMS substrate.
In the method of U.S. Pat. No. 6,401,526, the location of the nanotubes is determined by the placement of silicon pyramids. The silicon pyramids are placed at specific location, dip coated with a liquid phase catalyst, followed by chemical vapor deposition of carbon to make single-walled carbon nanotube probe-tips for atomic force microscopy. The nanotubes are then shortened to the desired length.
Thus, there is a need for methods for synthesizing individual carbon nanotubes at preselected location on a substrate. Preferably, the method allows for growth of a controlled number of carbon nanotubes at preselected locations on a substrate. In additon, the method also preferably allows for the growth of individual carbon nanotubes of a desired type, such as single-wall nanotubes.